If a 100 grain bullet isn't enough, why don't they say that about that same weight in the 243 or the 257 Roberts.

Because the 243 and the 257 have smaller bore diameters. 100 grains is a heavy bullet for a .243 bore, a medium bullet in a .257 bore and a very light (for big game) bullet for a .264 bore. It all has to do with sectional density, a function of bullet weight to bore diameter which a lot to do with how well the bullet penetrates.